The present invention relates to paper machines and in particular to the headboxes of paper machines.
In particular, the present invention relates to a hydraulic headbox of a paper machine, the pulp stock flowing in this headbox from a header thereof through a distribution pipe system to an equalizing chamber and from the latter through a turbulence passage system to the lip slice of the headbox. With the present invention, part of the above headbox structure cooperates with an air tank which contains air under pressure.
In general paper machine headboxes may be divided into three main groups, namely, (a) headboxes provided with an air cushion which communicates directly with the pulp stock in the headbox so as to provide a so-called air cushion headbox, (b) hydraulic headboxes provided with an air cushion separate from the headbox itself, where air tanks are located either at a part of the pipe system which delivers the pulp stock to the header of the headbox or at the headbox itself downstream of the header, for example, and (c) hydraulic headboxes which have no air cushion at all.
The use of an air cushion in connection with a headbox is intended to equalize pressure fluctuations occurring in the pulp suspension flowing toward the discharge aperture or lip slice of the headbox. Such pressure fluctuations may originate in the pulp stock flow system upstream of the headbox or at the headbox itself. In the event that these pressure fluctuations are permitted to proceed all the way up to the lip of the headbox, they will result in velocity variations in the discharging pulp jet, and the consequence is that base weight variations will occur in the pulp web which is formed on the forming wire. Such longitudinal base weight variations cannot be entirely eliminated, during subsequent drying of the web, and thus such variations will be visible in the finished paper, detracting from the value thereof.
In order to achieve a uniform average flow velocity profile in the cross-machine direction, the upstream part of the headbox, in the form of a distribution header, is usually of a tapered configuration, tapering in the direction of flow and having, for example, configuration of a truncated cone or the equivalent thereof. At the downstream end of such a distribution header there is often a continuous by-pass flow. From this header the pulp stock is delivered to a number of so-called diffuser pipes which are distributed transversely of the machine with a constant spacing and which extend longitudinally in the direction of pulp stock flow from the header toward the lip slice, so that through such a distribution pipe system the pulp stock is conveyed away from the header toward the lip slice.
With a headbox according to group (a) referred to above, the air cushion damps or attenuates pressure fluctuations in a highly efficient manner because with such a construction the surface area of the pulp stock contacted by the air cushion is comparatively large while the depth of the pulp stock where it is contacted by the air cushion is relatively small in a direction perpendicular to the direction of flow of the pulp stock. Such headboxes also have the advantage that the air cushion usually extends up to a location which is very close to the discharge lip of the headbox, so that at the region between the location of action of the air cushion and the lip slice the opportunity for generation of new pressure fluctuations is very low.
In spite of the above advantages of the air cushion type of headbox, the latter has not been used in recent times and has to a great extent been replaced in the newest fast paper machines by hydraulic or fully hydraulic headboxes respectively referred to above under groups (b) and (c). This replacement of the air cushion headboxes with hydraulic headboxes is due to the fact that the hydraulic headboxes are easier to situate with respect to the new twin-wire formers. Also, they are relatively inexpensive to manufacture. The greater turbulence of the pulp jet discharging from the lip of such headboxes and its more favorable intensity distribution as well as the improved homogeneity of the pulp stock have also favored the use of such hydraulic headboxes.
However, as opposed to the above advantages, hydraulic headboxes have difficulties with respect to the pressure fluctuations referred to above. Quite often a headbox initially meant to be fully hydraulic is required to be subsequently fitted with one or more separate air tanks which are intended to provide a substitute for the air cushion of an air cushion headbox. With respect to the location of such separate air tanks, various design solutions are known, and in some of them the air tanks have been connected to the pulp stock pipe system in advance of the headbox while in others the air tanks are connected above the headbox itself, the connection being provided by way of connecting tubes or ducts communicating with an upper part of the headbox. With respect to the first type of construction, there is the drawback that pressure fluctuations generated upstream of the air tank may indeed be sufficiently damped, but in the region between the air tank and the lip slice of the headbox there are new pressure fluctuations originating from various sources such as, for example, errors in the configuration of the distribution header, and such new pressure fluctuations spread without attenuation up to the lip slice of the headbox resulting in base weight variations of the paper as referred to above. With the latter type of construction there is the drawback that with an air tank situated above the headbox, the height of the free liquid from the central axis of liquid flow is relatively great, or the communicating tubes or ducts from the headbox to the air tank must be dimensioned so as to be too narrow as compared with the main flow duct. In both cases the damping characteristics are substantially impaired, as compared with the pressure fluctuation equalizing capacity of a standard air cushion headbox.